This invention relates to an electrochemical apparatus containing a plurality of electrochemical cells, each having an exterior fuel electrode, which can be contacted by a mixture of fresh feed fuel and recirculated spent fuel, which mixture contains water vapor and can pass through a reforming material prior to contacting the fuel electrodes, where recirculated, combusted exhaust gas surrounds and heats the reforming material, and fresh feed fuel and recirculated spent fuel can be mixed precisely by valving controls prior to contact with the cells.
High temperature, solid oxide electrolyte fuel cell generator apparatus and fuel cell configurations are well known, and taught, for example, in U.S. Pat. Nos. 4,395,468 and 4,490,444 respectively (Isenberg). There, feed fuel, either H.sub.2 +CO, or previously reformed natural gas, is fed into the apparatus at one end and flows parallel to exterior fuel electrode surfaces of elongated fuel cells. Spent fuel is combusted with spent oxidant in a separate chamber and then exits the apparatus.
Other high temperature fuel cell generator apparatus designs are known, where spent fuel is recirculated and aspirated into fresh, preheated feed fuel, in the center of the apparatus, at apparent temperatures of approximately 600.degree. C. to 800.degree. C., which mixture is fed through the fuel cells, as taught in U.S. Pat. No. 3,718,506 (Fischer et al.). There, spent fuel exiting from the fuel cells, is mixed with fresh feed fuel, such as propane, at a jet-pump nozzle, for flow inside of series connected fuel cells. This combination of spent fuel with fresh fuel prevents soot formation within the cells. Additional spent fuel mixes with spent air and flows around and through a catalytic afterburner structure surrounding the fuel cells for complete combustion, which heats the fuel cells, allowing efficient operation at approximately 800.degree. C. In U.S. Pat. No, 4,729,931 (Grimble), spent fuel and combusted fuel are mixed, and then drawn into fresh feed fuel at an ejector nozzle, in an interior chamber of the generator apparatus. The entire mixture then passes through a reforming material, located in the same chamber as the ejector, to form a reformed gas which is fed to contact fuel cells within the apparatus.
Another generator design to prevent chilling of the fuel cells is taught in U.S. Pat. No. 4,808,491 (Reichner) where a combusted exhaust of spent fuel and spent oxidant provides corner heating in the generator apparatus. There, fresh feed fuel is fed into a manifold at the bottom of the apparatus, the bottom of which manifold may contain reforming catalyst and may be heated by the combusted exhaust. The feed fuel then flows parallel to the exterior fuel electrode surfaces of the elongated fuel cells. The fresh feed fuel is not mixed with any spent gases.
Natural gas (methane plus ethane, propane, butane and nitrogen) is a likely fuel for many of these fuel cell apparatus. This natural gas must be reformed, that is, cornered to carbon monoxide and hydrogen, through the use of a catalyst and excess water vapor, prior to its utilization in the fuel cell. The reforming reaction is endothermic, requiring a supply of heat, and is best performed at temperatures close to 900.degree. C. The heat required for reforming is a significant fraction of the excess heat that results from fuel cell operation.
The use of recirculated spent fuel to provide water vapor and CO.sub.2 for fresh feed fuel, by means of an ejector powered by the inlet fresh fuel pressure, has the potential to result in several problems. In many instances, the composition of the fresh feed fuel may vary, requiring control of the amount passing through the ejector, and also control of the amount of water vapor laden spent fuel. Also, too great a draw of spent fuel at the ejector, will reduce the Nernst potential in a significant portion of the generating chamber of the apparatus and will result in lower operating voltage or poorer utilization of fuel than desired.
What is needed is an apparatus which allows more efficient heating of internal reforming material, and which controls spent fuel inclusion into the fresh feed fuel. It is one of the main objects of this invention to provide such an apparatus.